Two-phase polyimide-polyimide composite aerogel and preparation method thereof

Abstract

The invention relates to the technical field of aerogel, and discloses two-phase polyimide-polyimide composite aerogel and a preparation method thereof. The method comprises the following steps: (1) preparing polyimide gel microspheres; (2) carrying out first mixing on the polyimide gel microspheres and a polyimide solution II under a vacuum condition, and then sequentially carrying out preforming, standing and gelling to obtain two-phase composite polyimide wet gel; and (3) sequentially carrying out aging, solvent replacement and drying on the two-phase composite polyimide wet gel to obtain the two-phase polyimide-polyimide composite aerogel. The preparation method provided by the invention can effectively overcome the two-phase interface defect caused by conventional compounding, and the chemical structure and pore morphology of the polyimide microspheres are regulated and controlled to regulate and control the composite aerogel structure, so that the high-performance two-phase polyimide aerogel material with high hydrophobicity, low thermal conductivity, low density and excellent high-temperature dimensional stability is prepared.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, and particularly relates to a two-phase polyimide-polyimide composite aerogel and a preparation method thereof. Background Art

[0002] Polyimide is a class of high-performance materials with imide rings in the main chain structure. Due to its excellent high-temperature resistance, low-temperature resistance, mechanical properties, chemical stability, and low dielectric constant, it is widely used in high-tech fields such as aviation, aerospace, engineering plastics, and microelectronics. Polyimide aerogel is a porous material composed of cross-linked three-dimensional nanofibers formed by polymer molecular chains, combining the excellent properties of polyimide and aerogel, making it not only have the excellent characteristics of polyimide but also have outstanding features such as lightweight, ultra-low density, high specific surface area, low thermal conductivity, and low dielectric constant of aerogel. Compared with highly brittle inorganic aerogels and conventional organic aerogels with low thermal stability, polyimide aerogels have high mechanical properties, good thermal stability, and low thermal conductivity, which greatly expands their application scope, especially in the field of aerospace. For example, flexible polyimide aerogels are applied in the anti / thermal insulation systems of spacecraft, liquid hydrogen and liquid oxygen storage tanks of aircraft, submarine acoustic barrier systems, thermal protection systems for hypersonic inflatable pneumatic decelerators, spacesuits, and microstrip patch antennas, etc., promoting the application research of polyimide aerogel materials and attracting wide attention and development of polyimide aerogel materials.

[0003] Although the mesoporous structure with interlaced nanofibers of polyimide aerogel endows it with the above excellent properties, this mesoporous structure has poor stability in high-temperature and high-humidity environments, specifically manifested as the collapse of the mesoporous structure under high-temperature and high-humidity conditions, resulting in a decline in its performance. This greatly limits the use environment of polyimide aerogel and cannot fully utilize the high-temperature thermal stability of polyimide materials superior to conventional polymer materials.

[0004] Generally, the hydrophobicity and high-temperature stability of polyimide aerogels are solved by two methods. One is to use monomers with good hydrophobicity and high-temperature stability. For example, CN114854083A endows the material with excellent hydrophobic properties by introducing fluorine-containing monomers to ensure the long-term use stability of the material in high-humidity environments; CN113831582A prepares hydrophobic and high-temperature-resistant polyimide aerogels through fluorinated dianhydrides and siloxanes with dimethyl groups; CN109942848A uses cage-type low-functional amino phenyl silsesquioxane as a cross-linking agent to prepare superhydrophobic and high-temperature-resistant aerogels; CN106750493A adds NH 2Using -HBPSiF as a cross - linker, a polyimide aerogel with low water absorption was prepared. However, this method significantly changed the chain segment structure of the polyimide aerogel material, having a greater impact on its properties such as thermal conductivity and density.

[0005] Second, the method of adding fillers such as hydrophobic silica aerogel powder. For example, in CN108727818A, a hydrophobic modified silica aerogel filler was embedded in the pores of the polyimide aerogel, improving the high - temperature stability and to a certain extent improving the hydrophobicity of the polyimide aerogel. However, the effect was not ideal, and the particle size requirements for the hydrophobic aerogel powder were relatively strict, which must be between 10 - 20 nm. And if there was accumulation or aggregation of the doped material during the preparation of the aerogel, the stress concentration or the poor contact caused by the interfacial tension at the inorganic - organic interface would make it difficult for the modified aerogel material to be formed or its performance to decline.

[0006] At present, there is an urgent need to provide a method for preparing a polyimide composite aerogel with good hydrophobicity and high - temperature dimensional stability. Summary of the Invention

[0007] The purpose of the present invention is to solve the problems of the two - phase interface existing in conventional composite aerogels, as well as the poor hydrophobicity and high - temperature dimensional stability of polyimide composite aerogels.

[0008] To achieve the above - mentioned purpose, in the first aspect of the present invention, a method for preparing a two - phase polyimide - polyimide composite aerogel is provided. The method includes the following steps:

[0009] (1) Prepare polyimide gel microspheres

[0010] S1: Under the action of an air flow, atomize the polyimide solution I and then spray it into a coagulation bath to obtain polyimide wet gel microspheres;

[0011] S2: Subject the polyimide wet gel microspheres to aging, solvent replacement, and drying in sequence to obtain polyimide aerogel microspheres;

[0012] (2) After first mixing the polyimide gel microspheres and the polyimide solution II under vacuum conditions, then perform pre - forming, standing, and gelling in sequence to obtain a two - phase composite polyimide wet gel; the polyimide gel microspheres are the polyimide aerogel microspheres prepared in step (1) and / or the polyimide wet gel microspheres;

[0013] (3) Subject the two - phase composite polyimide wet gel to aging, solvent replacement, and drying in sequence to obtain a two - phase polyimide - polyimide composite aerogel;

[0014] Among them, the polyimide solution I is obtained by subjecting polyamic acid I to a cross-linking reaction and a chemical cyclization reaction, and the polyimide solution II is obtained by subjecting polyamic acid II to a cross-linking reaction and a chemical cyclization reaction, and the types of the polyamic acid I and the polyamic acid II are different; the polyamic acid I and the polyamic acid II contain structural units obtained by polycondensation of diamine monomers and dianhydride monomers, and the diamine monomers and / or the dianhydride monomers contain hydrophobic groups;

[0015] The hydrophobic group is selected from at least one of an alkyl group of C 1-3 an alkyl group of a halogen, and a halogen-substituted C 1-3 alkyl group;

[0016] In the polyamic acid I and the polyamic acid II, the molar ratio of the structural units provided by the diamine monomers to the structural units provided by the dianhydride monomers is n:(n + 1) or (n + 1):n, where n represents the degree of polymerization and n is an integer of 15-45.

[0017] The second aspect of the present invention provides a two-phase polyimide-polyimide composite aerogel prepared by the method described in the first aspect.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] (1) In the present invention, by compounding the prepared hydrophobic polyimide microspheres with the polyimide solution, both phases of the obtained composite aerogel are polyimide systems, eliminating interfacial defects;

[0020] (2) The polyimide aerogel microspheres prepared in the present invention have controllable particle sizes by controlling the air flow, which will not affect the formation of the two-phase composite polyimide aerogel and can meet the requirements for preparing blocks or films;

[0021] (3) In the present invention, using hydrophobic polyimide aerogel microspheres as fillers not only improves the hydrophobic performance of the system but also overcomes the defect that the mechanical properties of the product decrease when using silica particles as fillers in the prior art; meanwhile, by adjusting the microstructure of the fillers, the pore structure of the two-phase composite aerogel can be effectively regulated so that the two-phase composite aerogel has the advantage of high-temperature dimensional stability;

[0022] (4) The composite polyimide aerogel prepared in the present invention is suitable for industrial production. Detailed Embodiments

[0023] The endpoints and any values disclosed in this text for a range are not limited to the exact range or value. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this text.

[0024] As described above, the first aspect of the present invention provides a method for preparing a two-phase polyimide-polyimide composite aerogel, and the method includes the following steps:

[0025] (1) Prepare polyimide gel microspheres

[0026] S1: Under the action of an air flow, atomize the polyimide solution I and then spray it into a coagulation bath to obtain polyimide wet gel microspheres;

[0027] S2: Subject the polyimide wet gel microspheres to aging, solvent replacement, and drying in sequence to obtain polyimide aerogel microspheres;

[0028] (2) After first mixing the polyimide gel microspheres with the polyimide solution II under vacuum conditions, then perform preforming, standing, and gelation in sequence to obtain a two-phase composite polyimide wet gel; the polyimide gel microspheres are the polyimide aerogel microspheres prepared in step (1) and / or the polyimide wet gel microspheres;

[0029] (3) Subject the two-phase composite polyimide wet gel to aging, solvent replacement, and drying in sequence to obtain a two-phase polyimide-polyimide composite aerogel;

[0030] Wherein, the polyimide solution I is obtained by a cross-linking reaction and a chemical cyclization reaction of polyamic acid I, and the polyimide solution II is obtained by a cross-linking reaction and a chemical cyclization reaction of polyamic acid II, and the types of the polyamic acid I and the polyamic acid II are different; the polyamic acid I and the polyamic acid II contain structural units obtained by condensation polymerization of diamine monomers and dianhydride monomers, and the diamine monomers and / or the dianhydride monomers contain hydrophobic groups;

[0031] The hydrophobic group is selected from at least one of an alkyl group of C 1-3 halogen, and an alkyl group of C 1-3 substituted by halogen;

[0032] In the polyamic acid I and the polyamic acid II, the molar ratio of the structural unit provided by the diamine monomer to the structural unit provided by the dianhydride monomer is n:(n + 1) or (n + 1):n, where n represents the degree of polymerization and n is an integer of 15 - 45.

[0033] Preferably, when the polyimide gel microspheres are the polyimide wet gel microspheres prepared in step (1), before the first mixing in step (2), the polyimide wet gel microspheres are aged at 15 - 60 °C for 4 - 24 h, and then the aged polyimide wet gel microspheres are mixed with the polyimide solution II for the first time.

[0034] Preferably, the diamine monomer is selected from at least one of 4,4'-diaminodiphenyl ether (ODA), 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl (DMBZ), 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB), p-phenylenediamine (PPDA), and polyetheramine (PPG).

[0035] Preferably, the dianhydride monomer is selected from at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), pyromellitic dianhydride (PMDA), 4,4'-biphenylether dianhydride, and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA).

[0036] More preferably, in the polyamic acid solution I and / or the polyamic acid solution II, the monomer for providing a hydrophobic structural unit is selected from at least one of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 2,2'-bis(trifluoromethyl)diaminobiphenyl (TFMB), and polyetheramine (PPG). The inventors found that in this preferred case, the obtained two-phase polyimide-polyimide composite aerogel has better hydrophobic properties.

[0037] Preferably, in step (2), the polyimide gel microspheres are the polyimide wet gel microspheres and polyimide aerogel microspheres prepared in step (1), and the mass ratio of the polyimide wet gel microspheres to the polyimide aerogel microspheres is 1:0.1 - 1.

[0038] Preferably, the pressure of the air flow in step S1 is controlled to be 2 - 6 bar, so that the average particle size of the obtained polyimide wet gel microspheres is 0.5 - 50 μm. More preferably, the average particle size of the polyimide wet gel microspheres is 1 - 10 μm.

[0039] Preferably, in step S1, the coagulation bath consists of a poor solvent, and the poor solvent is selected from at least one of water, ethanol, and acetone.

[0040] In the present invention, the principle of forming the polyimide wet gel microspheres is that the polyimide solution I undergoes rapid phase separation in a poor solvent.

[0041] Preferably, the average particle size of the polyimide aerogel microspheres is 0.5-50 μm. More preferably, the average particle size of the polyimide wet gel microspheres is 1-10 μm.

[0042] Preferably, in step (2), the conditions for the first mixing include: the time is 1-5 min, and the stirring speed is 500-2500 rpm.

[0043] Preferably, in step (2), the mass of the polyimide gel microspheres accounts for 5-400 wt% of the polyimide solution II based on dry basis.

[0044] More preferably, in step (2), when the polyimide gel microspheres are the polyimide wet gel microspheres prepared in step (1), the mass of the polyimide gel microspheres accounts for 25-400 wt% of the polyimide solution II based on dry basis; when the polyimide gel microspheres are the polyimide aerogel microspheres prepared in step (1), the mass of the polyimide gel microspheres accounts for 5-20 wt% of the polyimide solution II based on dry basis. The inventors found that under these preferred conditions, it is more beneficial to improve the high-temperature dimensional stability of the polyimide-polyimide composite aerogel.

[0045] Preferably, before performing step S1, polyimide solution I is first prepared, and then the prepared polyimide solution I is atomized; the preparation method of the polyimide solution I includes:

[0046] S11: In the presence of a polar aprotic solvent, a diamine monomer and a dianhydride monomer are subjected to a polymerization reaction to obtain polyamic acid I;

[0047] Control the amounts of the diamine monomer and the dianhydride monomer such that the molar ratio of the structural units provided by the diamine monomer to the structural units provided by the dianhydride monomer in the polyamic acid I is n:(n + 1) or (n + 1):n, where n represents the degree of polymerization and n = 15-45;

[0048] S12: In the presence of a polar aprotic solvent, after the polyamic acid I is subjected to a crosslinking reaction with a crosslinking agent, and then subjected to a chemical cyclization reaction with a cyclizing agent, polyimide solution I is obtained; or

[0049] In the presence of a polar aprotic solvent, after the polyamic acid I is subjected to a chemical cyclization reaction with a cyclizing agent, and then subjected to a crosslinking reaction with a crosslinking agent, polyimide solution I is obtained.

[0050] Preferably, in steps S11 and S12, the polar aprotic solvent is selected from at least one of N,N-dimethylacetamide (DMAc), dimethyl sulfoxide, and N-methylpyrrolidone.

[0051] Preferably, the amount of the polar aprotic solvent is controlled such that the solid content of the polyimide solution I is 5 wt% - 20 wt%. It should be noted that the "amount of the polar aprotic solvent" defined in the present invention is the sum of the amount of the polar aprotic solvent in step S11 and the amount of the polar aprotic solvent in step S12.

[0052] Preferably, the crosslinking agent is selected from at least one of 1,3,5-benzenetricarbonyl trichloride (BTC), tris(2-aminoethyl)amine (TREN), 1,3,5-tris(4-aminophenoxy)benzene (TAB), and octakis(aminophenyl)silsesquioxane (OAPS).

[0053] More preferably, when the molar ratio of the structural unit provided by the diamine monomer to the structural unit provided by the dianhydride monomer is (n + 1):n, the crosslinking agent is 1,3,5-benzenetricarbonyl trichloride (BTC); when the molar ratio of the structural unit provided by the diamine monomer to the structural unit provided by the dianhydride monomer is n:(n + 1), the crosslinking agent is selected from at least one of tris(2-aminoethyl)amine (TREN), 1,3,5-tris(4-aminophenoxy)benzene (TAB), and octakis(aminophenyl)silsesquioxane (OAPS). The inventors have found that in this preferred case, the pore structure of the prepared polyimide solution I is more uniform during the formation of microspheres.

[0054] Preferably, relative to 1 mmol of the dianhydride monomer, the amount of the crosslinking agent is 0.005 - 0.05 mmol. The inventors have found that in this preferred case, the pore structure of the prepared polyimide solution I is more uniform during the formation of microspheres, which is beneficial to reducing the thermal conductivity of the polyimide-polyimide composite aerogel.

[0055] Preferably, in step S12, the cyclization reagent is a mixed solution of acetic anhydride and a basic organic solvent, and the molar ratio of acetic anhydride to the basic organic solvent in the mixed solution is 1:0.6 - 1. The basic organic solvent is selected from at least one of triethylamine and pyridine.

[0056] Preferably, relative to 1 mmol of the dianhydride monomer, the amount of acetic anhydride in the cyclization reagent is 3 - 8 mmol.

[0057] Preferably, in step S11, the conditions for the polymerization reaction include: the time is 30 - 120 min, and the stirring speed is 300 - 800 rpm.

[0058] Preferably, in step S12, the conditions for the crosslinking reaction include: the time is 1 - 10 min, and the stirring speed is 300 - 800 rpm.

[0059] Preferably, in step S12, the conditions for the chemical cyclization reaction include: a time of 1 - 10 min and a stirring speed of 300 - 800 rpm.

[0060] In the present invention, in the case where the reaction temperature is not specified for each reaction, it means that the reaction is carried out under room temperature conditions, and room temperature means 25 ± 1 °C.

[0061] In the present invention, the polyimide solution II in step (2) can be prepared by a method similar to that of the polyimide solution I or can be obtained by purchase.

[0062] The present invention does not particularly limit the preforming method in step (2). It can be formed into a block in a mold or made into a film shape in a coating machine, and those skilled in the art can select according to needs.

[0063] The present invention does not particularly limit the specific time for standing and gelling in step (2), as long as the purpose of fully gelling the solution after the first mixing can be achieved.

[0064] Preferably, in steps S2 and (3), the conditions for aging include: a temperature of 15 - 60 °C and a time of 4 - 24 h.

[0065] Preferably, in steps S2 and (3), the solvent replacement is carried out by means of solvent gradient replacement.

[0066] Preferably, the solvent replacement is carried out in four times, and the solvents used for the first and second solvent replacements are a mixed solution formed by one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide and solvent A, and the solvents used for the third and fourth solvent replacements are at least one of acetone and ethanol; the solvent A is acetone and / or ethanol.

[0067] Preferably, the total time for the solvent replacement is 24 - 96 h.

[0068] Preferably, in steps S2 and (3), the drying is independently selected from at least one of supercritical CO 2 drying and atmospheric drying.

[0069] Preferably, the conditions for the supercritical CO 2 drying include: a temperature of 45 - 70 °C, a pressure of 7.5 - 15 MPa, and a drying time of 0.5 - 8 h; the conditions for the atmospheric drying include: a temperature of 60 - 100 °C and a drying time of 1 - 6 h.

[0070] The preparation method of the two-phase polyimide-polyimide composite aerogel provided by the present invention can effectively overcome the two-phase interface defects brought by conventional composite methods, and can regulate the structure of the composite aerogel by adjusting the chemical structure and pore morphology of the polyimide microspheres on the basis of maintaining the original chain segment structure of the polyimide aerogel, thereby obtaining a two-phase composite aerogel material with high hydrophobicity, low thermal conductivity, low density, and excellent high-temperature dimensional stability.

[0071] As described above, the second aspect of the present invention provides a two-phase polyimide-polyimide composite aerogel prepared by the method described in the first aspect.

[0072] The present invention will be described in detail below through examples. In the following examples, unless otherwise specified, the reagents used are commercially available chemical reagents, and room temperature in the examples means 25±1°C.

[0073] 4,4'-Diaminodiphenyl ether (ODA), purchased from Macklin Biochemical Technology Co., Ltd., with a purity of ≥98%;

[0074] Polyetheramine (PPG), CAS: 9046-10-0, with an average molecular weight of 400, purchased from Sigma-Aldrich.

[0075] The thermal conductivity of the two-phase polyimide-polyimide composite aerogel prepared in the following examples was tested according to the GB / T10295 standard.

[0076] The water absorption rate was tested as follows: The prepared product was cut into a sample with dimensions of 1 cm×1 cm×0.5 cm, and the weight of the sample was weighed and recorded as m 0 ; then the sample was completely immersed in water for 24 h, taken out, the surface moisture was wiped off, and its weight was weighed and recorded as m 1 ; the water absorption rate was calculated through the water absorption rate formula;

[0077] Water absorption rate = (m 1 - m 0 ) / m 0 ×100%.

[0078] The volume shrinkage rate was tested as follows: The length, width, and height of the test sample were measured and recorded as l 0 , d 0 , h 0 , the sample was placed in a muffle furnace and heated to the test temperature of 300°C and held for 30 min, then the sample was taken out, and the length, width, and height of the sample were measured again and recorded as l 1 , d 1 , h 1 ;

[0079] Volume shrinkage rate = (l 0×d 0 ×h 0 -l 1 ×d 1 ×h 1 ) / (l 0 ×d 0 ×h 0 )×100%.

[0080] In Examples 1-4, polyimide solution I and polyimide solution II were prepared according to the formulations in Table 1.

[0081] Example 1

[0082] In this example, polyimide solution I and polyimide solution II were prepared according to the raw materials and dosages in Table 1;

[0083] (1) The diamine monomer I was added to the polar aprotic solvent I, and after stirring until completely dissolved, the dianhydride monomer I was added, and stirred at 500 rpm for 60 min to obtain polyamic acid I;

[0084] The cyclization reagent I was slowly added to the polyamic acid I, and mixed and stirred at 600 rpm for 3 min under vacuum, then the crosslinking reagent I and the polar aprotic solvent I were added, and stirred at 600 rpm for another 5 min to obtain polyimide solution I;

[0085] (2) The polyimide solution I was sprayed into a mixed solution (5 L) of water and ethanol (volume ratio 1:1) under the action of an air compressor pump (5 bar) to obtain polyimide wet gel microspheres. Then, after aging the polyimide wet gel microspheres at room temperature for 18 h, they were successively replaced with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, each replacement for 8 h, and then dried in an oven at 60 °C for 3 h to obtain polyimide aerogel microspheres for standby;

[0086] The average particle size of the prepared polyimide wet gel microspheres was 3 μm, and the average particle size of the polyimide aerogel microspheres was 2 μm;

[0087] (3) The diamine monomer II was added to the polar aprotic solvent II, and after stirring until completely dissolved, the dianhydride monomer II was added, and stirred at 400 rpm for 30 min to obtain polyamic acid II;

[0088] The crosslinking reagent II and the polar aprotic solvent II were added to the polyamic acid II, and after mixing and stirring at 600 rpm for 7 min, the cyclization reagent II was continuously added, and stirred and mixed at 600 rpm under vacuum for 3 min to obtain polyimide solution II;

[0089] (4) Mix the polyimide aerogel microspheres with polyimide solution II under vacuum at 2000 rpm for 3 min, then transfer to a film coater to prepare a 150-μm-thick film. After standing for gelation, age the film at room temperature for 20 h. Then, sequentially displace the film with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, with each displacement lasting 10 h. After solvent displacement is completed, dry it in a supercritical CO 2 (55 °C, 15 MPa) manner for 4 h to obtain a two-phase polyimide-polyimide composite aerogel film S1;

[0090] The mass of the polyimide aerogel microspheres accounts for 5 wt% of polyimide solution II based on dry basis;

[0091] The density of the obtained composite aerogel film S3 is 0.089 g / cm 3 , the thermal conductivity is 21 mW / (m·K), the water absorption rate in 24 h is 2.5%, and after heat treatment at 300 °C for 24 h, the volume shrinkage rate is 12.5%.

[0092] Example 2

[0093] In this example, polyimide solution I and polyimide solution II are prepared according to the raw materials and dosages in Table 1;

[0094] (1) Add diamine monomer I to polar aprotic solvent I, stir until completely dissolved, then add dianhydride monomer I, and stir at 800 rpm for 40 min to obtain polyamic acid solution I;

[0095] Slowly add cyclization reagent I to polyamic acid I, mix and stir under vacuum at 800 rpm for 2 min. Then add crosslinking reagent I and polar aprotic solvent I, and continue to stir at 800 rpm for 5 min to obtain polyimide solution I;

[0096] (2) Spray polyimide solution I into a mixed solution (5 L) of water and acetone (volume ratio 2:1) under the action of an air compressor pump (2 bar) to obtain polyimide wet gel microspheres. Then, age the polyimide wet gel microspheres at room temperature for 24 h, and sequentially displace them with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, with each displacement lasting 8 h. Then, dry it by supercritical CO 2 (45 °C, 15 MPa) for 3 h to obtain polyimide aerogel microspheres for standby;

[0097] The average particle size of the prepared polyimide wet gel microspheres is 7.5 μm, and the average particle size of the polyimide aerogel microspheres is 5 μm;

[0098] (3) Add diamine monomer II to polar aprotic solvent II, stir until completely dissolved, then add dianhydride monomer II, and stir at 700 rpm for 30 min to obtain polyamic acid II;

[0099] Add cyclization reagent II to polyamic acid II, stir and mix under vacuum at 700 rpm for 3 min, then add crosslinking reagent II and polar aprotic solvent II, and mix and stir at 700 rpm for 7 min to obtain polyimide solution II;

[0100] (4) Take polyimide aerogel microspheres and polyimide solution II, mix them at high speed under vacuum at 2000 rpm for 3 min, then transfer them to a mold and let them stand for 18 h, and then age them at room temperature for 24 h; successively displace the aged wet gel with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, each displacement for 8 h; after the solvent displacement is completed, dry the wet gel by supercritical CO 2 supercritical (55 °C, 10 MPa) for 5 h to obtain a blocky two-phase polyimide-polyimide composite aerogel S2;

[0101] The mass of the polyimide aerogel microspheres accounts for 15 wt% of the polyimide solution II based on dry basis;

[0102] The density of the obtained composite aerogel S2 is 0.11 g / cm 3 , the thermal conductivity is 22 mW / (m·K), the water absorption rate in 24 h is 2%, and after being treated at 300 °C for 24 h, the volume shrinkage rate is 12%.

[0103] Example 3

[0104] In this example, polyimide solution I and polyimide solution II were prepared according to the raw materials and dosages in Table 1;

[0105] (1) Add diamine monomer I to polar aprotic solvent I, stir until completely dissolved, then add dianhydride monomer I, and stir at 500 rpm for 50 min to obtain polyamic acid I;

[0106] Slowly add cyclization reagent I to polyamic acid I, stir and mix under vacuum at 400 rpm for 3 min, then add crosslinking reagent I and polar aprotic solvent I, and mix and stir at 500 rpm for 5 min to obtain polyimide solution I;

[0107] (2) The polyimide solution I was sprayed into a mixed solution (5 L) of acetone and ethanol (volume ratio 7:1) under the action of an air compressor pump (6 bar) to obtain polyimide wet gel microspheres. Then, after aging the polyimide wet gel microspheres at room temperature for 24 h, a part was set aside for use; the remaining part was successively replaced with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, with each replacement for 8 h, and then placed into 2 supercritical CO₂ (50 °C, 10 MPa) for drying for 6 h to obtain polyimide aerogel microspheres for standby;

[0108] The average particle size of the prepared polyimide wet gel microspheres was 7 μm, and the average particle size of the polyimide aerogel microspheres was 4.5 μm;

[0109] (3) The diamine monomer II was added to the polar aprotic solvent II, and after stirring until completely dissolved, the dianhydride monomer II was added, and stirred at 500 rpm for 30 min to obtain polyamic acid II;

[0110] Crosslinking reagent II and polar aprotic solvent II were added to the polyamic acid II. After stirring at 500 rpm for 10 min, a mixed solution of cyclization reagent II was added, and stirred under vacuum at 500 rpm for 4 min to obtain polyimide solution II;

[0111] (4) Polyimide aerogel microspheres and polyimide wet gel microspheres were taken and added to the polyimide solution II, mixed at 2000 rpm under vacuum for 3 min, and then transferred to a film coater to prepare a 120-μm film. After standing for gelation, the film was aged at room temperature for 24 h; the aged film was successively replaced with a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone and absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, with each replacement for 15 h; after the solvent replacement was completed, it was dried at normal pressure (60 °C, 6 h) to obtain a two-phase polyimide-polyimide composite aerogel film S4;

[0112] The total mass of the polyimide gel microspheres accounted for 66 wt% of the polyimide solution II based on dry basis, and the mass ratio of the polyimide wet gel microspheres to the polyimide aerogel microspheres was 1:0.1;

[0113] The density of the obtained composite aerogel film S4 was 0.14 g / cm 3 , the thermal conductivity was 24 mW / (m·K), the water absorption rate in 24 h was 3%, and the volume shrinkage rate after being treated at 300 °C for 24 h was 10%.

[0114] Example 4

[0115] In this example, polyimide solution I and polyimide solution II were prepared according to the raw materials and dosages in Table 1.

[0116] (1) Diamine monomer I was added to polar aprotic solvent I. After stirring until completely dissolved, dianhydride monomer I was added, and the mixture was stirred at 700 rpm for 50 min to obtain polyamic acid I.

[0117] Cyclization reagent I was slowly added to polyamic acid I, and the mixture was stirred under vacuum at 700 rpm for 3 min. Then crosslinking reagent I and polar aprotic solvent I were added, and the mixture was continuously stirred for 5 min to obtain polyimide solution I.

[0118] (2) The polyimide solution was sprayed into a mixed solution (5 L) of acetone and ethanol (volume ratio 7:1) under the action of an air compressor pump (6 bar) to obtain polyimide wet gel microspheres. Then the polyimide wet gel microspheres were aged at room temperature for 24 h and set aside.

[0119] The average particle size of the prepared polyimide wet gel microspheres was 1 μm.

[0120] (3) Diamine monomer II was added to polar aprotic solvent II. After stirring until completely dissolved, 15 mmol of dianhydride monomer II was added, and the mixture was stirred at 800 rpm for 30 min to obtain polyamic acid II.

[0121] Crosslinking reagent II and polar aprotic solvent II were added to polyamic acid II. After stirring at 800 rpm for 8 min, cyclization reagent II was continuously added, and the mixture was stirred under vacuum at 800 rpm for 3 min to obtain polyimide solution II.

[0122] (4) Polyimide wet gel microspheres and polyimide solution II were mixed under vacuum at 2000 rpm for 3 min, and then transferred to a film coater to prepare a 300-μm-thick film. After standing for gelation, the film was aged at room temperature for 16 h. The aged film was successively replaced with a mixed solution of N-methylpyrrolidone-absolute ethanol with a volume ratio of 3:1, a mixed solution of N-methylpyrrolidone-absolute ethanol with a volume ratio of 1:3, absolute ethanol, and absolute ethanol, each replacement for 6 h. After the solvent replacement was completed, supercritical drying was carried out with CO 2 at 55 °C and 15 MPa for 8 h to obtain a two-phase polyimide-polyimide composite aerogel film S4.

[0123] The mass of the polyimide wet gel microspheres accounted for 400 wt% of the polyimide solution II based on dry basis.

[0124] The density of the obtained composite aerogel film S4 was 0.07 g / cm 3, the thermal conductivity is 19 mW / (m·K), the water absorption rate in 24 h is 3%, and the volume shrinkage rate is 12% after being treated at 300 °C for 24 hours.

[0125] Table 1

[0126]

[0127]

[0128] Example 5

[0129] In this example, a two-phase polyimide-polyimide composite aerogel was prepared by a method similar to that of Example 1. The difference is that in step (1), diamine monomer I is 14.43 mmol of polyetheramine (PPG);

[0130] In step (3), diamine monomer II is 14.47 mmol of 2,2'-bis(trifluoromethyl)benzidine (TFMB); dianhydride monomer II is 15 mmol of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA);

[0131] The remaining steps are the same as those in Example 1; a two-phase polyimide-polyimide composite aerogel S5 is prepared;

[0132] The average particle size of the prepared polyimide wet gel microspheres is 4 μm, and the average particle size of the polyimide aerogel microspheres is 2.5 μm;

[0133] The density of the obtained composite aerogel film S5 is 0.090 g / cm 3 , the thermal conductivity is 22 mW / (m·K), the water absorption rate in 24 h is 1.5%, and after heat treatment at 300 °C for 24 hours, the volume shrinkage rate is 11.5%.

[0134] Example 6

[0135] In this example, a two-phase polyimide-polyimide composite aerogel was prepared by a method similar to that of Example 1. The difference is that in step (2), the pressure of the air compression pump is 7 bar;

[0136] The remaining steps are the same as those in Example 1; a two-phase polyimide-polyimide composite aerogel S6 is prepared;

[0137] The average particle size of the prepared polyimide wet gel microspheres is 0.9 μm, and the average particle size of the polyimide aerogel microspheres is 0.6 μm;

[0138] The density of the obtained composite aerogel film S6 is 0.1 g / cm 3 , the thermal conductivity is 26 mW / (m·K), the water absorption rate in 24 h is 2.8%, and after heat treatment at 300 °C for 24 hours, the volume shrinkage rate is 16%.

[0139] Example 7

[0140] In this example, a two-phase polyimide-polyimide composite aerogel was prepared by a method similar to that of Example 1. The difference is that in step (4), the mass of the polyimide aerogel microspheres accounts for 22 wt% of the polyimide solution II based on dry basis;

[0141] The remaining steps are the same as those in Example 1; a two-phase polyimide-polyimide composite aerogel S7 was obtained;

[0142] The average particle size of the prepared polyimide wet gel microspheres is 30 μm, and the average particle size of the polyimide aerogel microspheres is 25 μm;

[0143] The density of the obtained composite aerogel film S7 is 0.15 g / cm 3 , the thermal conductivity is 27 mW / (m·K), the water absorption rate in 24 h is 3%, and after heat treatment at 300 °C for 24 hours, the volume shrinkage rate is 10%.

[0144] Comparative Example 1

[0145] In this example, a two-phase polyimide-polyimide composite aerogel was prepared by a method similar to that of Example 1. The differences are as follows:

[0146] In step (1), diamine monomer I is 14.43 mmol of 4,4'-diaminodiphenyl ether (ODA); dianhydride monomer I is 15 mmol of 4,4'-biphenyl ether dianhydride;

[0147] In step (3), diamine monomer II is 14.47 mmol of p-phenylenediamine; dianhydride monomer II is 15 mmol of 4,4'-biphenyl ether dianhydride;

[0148] The remaining steps are the same as those in Example 1; a two-phase polyimide-polyimide composite aerogel D2 was obtained;

[0149] The average particle size of the prepared polyimide wet gel microspheres is 5 μm, and the average particle size of the polyimide aerogel microspheres is 3 μm;

[0150] The density of the obtained composite aerogel film D2 is 0.14 g / cm 3 , the thermal conductivity is 28 mW / (m·K), the water absorption rate in 24 h is 50%, and after heat treatment at 300 °C for 24 hours, the volume shrinkage rate is 12%.

[0151] Comparative Example 2

[0152] In this example, a two-phase polyimide-polyimide composite aerogel was prepared by a method similar to that of Example 1. The difference is that in step (1), the amount of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride is 16 mmol, and the degree of polymerization n in polyamic acid I is 9;

[0153] The remaining steps are the same as those in Example 1; a two-phase polyimide-polyimide composite aerogel D1 was prepared;

[0154] The average particle size of the prepared polyimide wet gel microspheres is 2 μm, and the average particle size of the polyimide aerogel microspheres is 1 μm;

[0155] In this comparative example, during the preparation of the composite aerogel film, due to the too small molecular weight of polyimide solution I, the viscosity during film coating was too low and it could not be formed.

[0156] From the results of the above examples, it can be seen that the two-phase polyimide-polyimide composite aerogel prepared by the method of the present invention has the advantages of high hydrophobicity, low thermal conductivity, and high-temperature dimensional stability.

[0157] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. Preparation method of a two-phase polyimide-polyimide composite aerogel, characterized in that, this method comprises the following steps: (1) Preparation of polyimide gel microspheres S1: Under the action of an air flow, the polyimide solution I is atomized and then sprayed into a coagulation bath to obtain polyimide wet gel microspheres; S2: The polyimide wet gel microspheres are sequentially aged, solvent-exchanged, and dried to obtain polyimide aerogel microspheres; (2) After the polyimide gel microspheres and the polyimide solution II are first mixed under vacuum conditions, they are then preformed, allowed to stand, and gelled in sequence to obtain a two-phase composite polyimide wet gel; the polyimide gel microspheres are the polyimide aerogel microspheres prepared in step (1) and / or the polyimide wet gel microspheres; (3) The two-phase composite polyimide wet gel is sequentially aged, solvent-exchanged, and dried to obtain a two-phase polyimide-polyimide composite aerogel; wherein, the polyimide solution I is obtained by cross-linking reaction and chemical cyclization reaction of polyamic acid I, and the polyimide solution II is obtained by cross-linking reaction and chemical cyclization reaction of polyamic acid II, and the types of polyamic acid I and polyamic acid II are different; the polyamic acid I and the polyamic acid II contain structural units obtained by condensation polymerization of diamine monomers and dianhydride monomers, and the diamine monomers and / or the dianhydride monomers contain hydrophobic groups; The hydrophobic group is selected from at least one of an alkyl group having C 1-3 , a halogen, and a halogen-substituted alkyl group having C 1-3 ; In the polyamic acid I and the polyamic acid II, the molar ratio of the structural units provided by the diamine monomers to the structural units provided by the dianhydride monomers is n:(n + 1) or (n + 1):n, where n represents the degree of polymerization and n is an integer from 15 to 45.

2. The method according to claim 1, wherein, the diamine monomers are selected from at least one of 4,4'-diaminodiphenyl ether, 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl, 2,2'-bis(trifluoromethyl)diaminobiphenyl, p-phenylenediamine, and polyetheramine; and / or the dianhydride monomers are selected from at least one of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenone tetracarboxylic dianhydride, pyromellitic dianhydride, 4,4'-biphenylether dianhydride, and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride; and / or In step (2), the polyimide microspheres are the polyimide wet gel microspheres and polyimide aerogel microspheres prepared in step (1), and the mass ratio of the polyimide wet gel microspheres to the polyimide aerogel microspheres is 1:0.1 - 1.

3. The method according to claim 1 or 2, wherein, in the polyamic acid solution I and / or the polyamic acid solution II, the monomers for providing hydrophobic structural units are selected from at least one of 4,4'-(hexafluoroisopropylidene)diphthalic anhydride, 2,2'-bis(trifluoromethyl)diaminobiphenyl, and polyetheramine; and / or control the pressure of the air flow in step S1 to be 2 - 6 bar so that the average particle size of the obtained polyimide wet gel microspheres is 0.5 - 50 μm; and / or In step S1, the coagulation bath consists of a poor solvent selected from at least one of water, ethanol, and acetone; and / or In step S2, the average particle size of the polyimide aerogel microspheres is 0.5 - 50 μm, and / or In step (2), the conditions for the first mixing include: the time is 1 - 5 min, and the stirring speed is 500 - 2500 rpm.

4. The method according to any one of claims 1 - 3, wherein, In step (2), the mass of the polyimide gel microspheres accounts for 5 - 400 wt% of the polyimide solution II based on dry basis; Preferably, when the polyimide gel microspheres are the polyimide wet gel microspheres prepared in step (1), the mass of the polyimide gel microspheres accounts for 25 - 400 wt% of the polyimide solution II based on dry basis; Preferably, when the polyimide gel microspheres are the polyimide aerogel microspheres prepared in step (1), the mass of the polyimide gel microspheres accounts for 5 - 20 wt% of the polyimide solution II based on dry basis.

5. The method according to any one of claims 1 - 4, wherein, Before step S1 is carried out, polyimide solution I is first prepared, and then the prepared polyimide solution I is atomized; The preparation method of the polyimide solution I includes: S11: In the presence of a polar aprotic solvent, a diamine monomer and a dianhydride monomer are subjected to a polymerization reaction to obtain polyamic acid I; The amounts of the diamine monomer and the dianhydride monomer are controlled such that the molar ratio of the structural units provided by the diamine monomer to the structural units provided by the dianhydride monomer in the polyamic acid I is n:(n + 1) or (n + 1):n, where n represents the degree of polymerization and n = 15 - 45; S12: In the presence of a polar aprotic solvent, after the polyamic acid I is crosslinked with a crosslinking agent, it is then subjected to a chemical cyclization reaction with a cyclizing agent to obtain polyimide solution I; or In the presence of a polar aprotic solvent, after the polyamic acid I is chemically cyclized with a cyclizing agent, it is then crosslinked with a crosslinking agent to obtain polyimide solution I.

6. The method according to claim 5, wherein, The crosslinking agent is selected from at least one of 1,3,5 - benzenetricarbonyl trichloride, tris(2 - aminoethyl)amine, 1,3,5 - tris(4 - aminophenoxy)benzene, octakis(aminophenyl)silsesquioxane; and / or Relative to 1 mmol of the dianhydride monomer, the amount of the crosslinking agent used is 0.005 - 0.05 mmol.

7. The method according to claim 5 or 6, wherein, In step S12, the cyclizing agent is a mixed solution of acetic anhydride and a basic organic solvent, and the molar ratio of acetic anhydride to the basic organic solvent in the mixed solution is 1:0.6 - 1, and the basic organic solvent is selected from at least one of triethylamine and pyridine; Preferably, relative to 1 mmol of the dianhydride monomer, the amount of acetic anhydride used in the cyclizing agent is 3 - 8 mmol.

8. The method according to any one of claims 5 - 7, wherein, In step S11, the conditions for the polymerization reaction include: a time of 30 - 120 min and a stirring speed of 300 - 800 rpm; and / or In step S12, the conditions for the cross-linking reaction include: a time of 1 - 10 min and a stirring speed of 300 - 800 rpm; and / or In step S12, the conditions for the chemical cyclization reaction include: a time of 1 - 10 min and a stirring speed of 300 - 800 rpm; and / or In steps S2 and (3), the conditions for the aging include: a temperature of 15 - 60 °C and a time of 4 - 24 h.

9. The method according to any one of claims 1 - 8, wherein, in steps S2 and (3), the solvent replacement is carried out in a solvent gradient replacement manner; preferably, the solvent replacement is carried out in four times, and the solvents used for the first and second solvent replacements are a mixed solution formed by one of N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide and solvent A, and the solvents used for the third and fourth solvent replacements are at least one of acetone and ethanol; the solvent A is acetone and / or absolute ethanol.

10. A two-phase polyimide-polyimide composite aerogel prepared by the method according to any one of claims 1 - 9.

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